The invention relates to a multistage canned motor pump, and more particularly to a multistage canned motor pump provided between a motor section and a pump section with a balance disk for proper control of a thrust balance.
It has been known in the prior art for proper control of the thrust balance multistage canned motor pumps utilize either an automatic balance feature or a self balance system in which impellers are provided to face each other.
FIG. 1 illustrates the conventional multistage canned motor pump with the automatic balance feature in which a multistage canned motor pump 10 includes a plurality of pump chambers 13 provided in individual stages of the pump. Each of the pump chambers 13 has an impeller 16 being provided with a balance hole 18 and a fixed orifice 20 at a rear side of the impeller so as to control the thrust balance.
The self balance system in which impellers are provided to face each other is disclosed in the Japanese Patent Publication No. 4-51678.
In the multistage canned motor pump having the automatic balance feature as illustrated in FIG. 1, a pressure of the last stage pump chamber 13a in the pump section 12 is directly applied to a chamber of the motor section 14. For that reason, the structure of the rotor chamber is designed in its strength to bear a pressured raised up by the final stage of the pump section 12.
Increase of the number of the stages results in a high head thereby designing the pump to bear a high pressure is necessary. This requires a thick can or a thick backup sleeve. The driving motor is required to bear the high pressure. Driving the motor capable of bearing the high pressure requires a large current thereby resulting in an increase of an energy loss. This therefore results in a lowering of the pump efficiency.
In the multistage canned motor pump with the self balance system, the rotor chamber is applied with a pressure of the intermediate stage of the pump section. However, in a large number of the stages of the pump section, the rotor chamber has to be designed to bare the high pressure. Further, it is required to provide a flow passage for introducing a treating liquid into a reverse impeller facing to a counterpart impeller. This makes the structure of the pump section complicated thereby resulting in an increase of the manufacturing cost.
In the conventional multistage pump with the balance disk, the balance disk 22 at its side abutting to a behind chamber 24 is connected through a circulation system 26 to a pump inlet port side chamber 25a to keep a low pressure of the pump section as illustrated in FIGS. 2A and 2B.
Normally, it is preferable to provide a circulation system in the multistage canned motor pump for cooling the motor section and lubrication of the bearings in which the circulation system has the same direction flow as that of the balance disk as providing a simple structure rather than the reverse direction.
The canned motor pump has the advantage of no leakage. Providing the circulation system at an exterior of the pump section requires an increase of the number of the sealing members. This provides an increase of a possibility of generation of the leakage. To combat this problem, as illustrated in FIG. 3, mainly used is a single stage canned motor pump 30 in which a through hole 34 is provided on a motor rotor 32 to permit a part of the treating liquid is circulated into a pump inlet port 25 or to establish an internal circulation system.
In applying such the system to the multistage canned motor pump with the balance disk, the following problems are generated. In the multistage canned motor pump, the pump axis and the motor axis are united to form a single motor rotor thereby resulting in a large longitudinal length of the canned motor pump. A diameter of the through hole on the shaft depends upon a strength of the shaft thereby a pressure loss due to the through hole is increased and then a pressure of the behind chamber provided behind the balance disk is raised up. This makes it difficult to keep the balance.
The internal circulation system may provide advantages in less leakage and a reduced manufacturing cost. The thrust balance feature using the balance disk may implement the stable balance over the all flow region and require no balance hole nor annular sealing member in the rear side. This may reduce the leakage loss and the disk friction loss to improve the pump efficiency. Then, the internal circulation system using the balance disk system has been used.
In a one-side suction multistage pump, a pump thrust directed to the section side is generated. To prevent this bias of the thrust, a rotation balance disk is normally used for the thrust balance feature.
In the thrust balance feature of FIG. 4, is bypassed through a high pressure chamber 46 and a low pressure chamber 48 to the low pressure suction port wherein the high and low pressure chambers are separated through an aperture 44 with a gap g between a rotation balance disk 40 and a balance sheet 42. The aperture 44 provides a pressure drop by which a balance between a balance thrust T.sub.B generated between the balance chambers 46 and 48 and the pump thrust T.sub.P.
Increase of the pump thrust T.sub.P makes the shaft 15 move in the pump thrust direction thereby the gap g is reduced. This may increase the pressure drop due to the aperture 44 thereby the balance thrust T.sub.B is increased. By contrast, when the pump thrust T.sub.P is reduced, the rotor shaft 15 moves to the balance thrust thereby the gap g is increased. This provides a reduction of the pressure drop due to the aperture 44 thereby a balance thrust T.sub.B is reduced. Then, the rotor shaft 15 moves automatically to such a position that both the pump thrust T.sub.P and the balance thrust T.sub.B are the same as each other thereby the both thrusts are set off. The thrusts T.sub.P and T.sub.B are given by the following equation (1) and illustrated in FIG. 5. ##EQU1## where P.sub.46' and P.sub.48' are high and low pressures of the high and low pressure chambers 46 and 48 respectively, r.sub.B and r.sub.S are the outer and inner diameters of the balance disk 40. p.sub.1', p.sub.2' and p.sub.3' are pressure at the position I of the inner diameter of the high pressure balance chamber 46, at the position II of the output port of the aperture 44 and at the position III of the outer diameter of the low pressure balance chamber 48.
The above conventional thrust balance feature is still engaged with problems as described below. Normally, the multistage pump requires the high head and the large capacity. The high head and the large capacity requires the increase of the number of the stages in the pump section thereby the pump thrust is also increased. To keep the balance, the balance thrust is required to be increased. The increase of the balance thrust needs specific design modifications thereby the manufacturing cost and the friction loss are increased.
In the structure as illustrated in FIG. 4, increase of the pump thrust T.sub.p causes a reduction of the gap g of the aperture 44 between the balance disk 40 and the balance sheet 42. In the conventional thrust balance feature, when the high head and large capacity pump is driven in a relatively large flow rate and in a low head region, the gap g is reduced excessively so that the balance disk 40 and the balance sheet 42 are made contact with each other. To prevent this problem, it is required to enlarge the outer diameter r.sub.B of the balance disk 40.
Such method of enlargement of the outer diameter is carried out by an enlargement of the both balance chambers 46 and 48 including the balance disk 40 and the balance sheet 42 and other elements. This results in an increase of the manufacturing cost and in a lowering of the pump efficiency. Notwithstanding, the minimization of the sizes of the pump constitutional elements such as the balance disk is preferable. The above problems are caused by an insufficient balance thrust due to a pressure generation feature of the conventional thrust balance feature. It is therefore required to solve the problem.
The description will focus on the structure and operations of the pressure generation feature. With reference to FIG. 4, in a space S between the both chambers 46 and 48, a rotation flow of the treating liquid is generated due to the rotation of the balance disk 40. The rotation flow may be regarded as a compulsory swirl flow relative to the rotation speed of the balance disk. The rotation flow is given by the following two equations. EQU u=Kr.omega.
where u is the peripheral speed of the treating liquid, r is the radius, .omega. is the angular speed of the balance disk and K is the specific peripheral speed. EQU p=.rho./2.times.K.sup.2 (r.sub.o.sup.2 -r.sub.i.sup.2).omega..sup.2
where .DELTA.p is the difference in pressure in the space S, .rho. is the density of the treating liquid, r.sub.o is the outer radius and r.sub.i is the inner radius.
From the above, it could be understood that the specific peripheral speed K is given by the function of the space S and the outer diameter r.sub.B. It has been known in the art that the value of the specific peripheral speed K is in the range of from 0.5 to 0.4 in the space as illustrated in FIG. 4.
In FIG. 5, both components of the balance thrust T.sub.B generated in the both balance chambers 46 and 48, or both pressures P.sub.46' and P.sub.48' are represented by areas P.sub.46' and P.sub.48' defined by lines L.sub.46' and L.sub.48' and a vertical axis r. The balance thrust T.sub.B is large as the high pressure balance chamber 46 has a large pressure P.sub.46' and the low pressure balance chamber 48 has a low pressure P.sub.48'.
In the conventional thrust balance feature, it is difficult to enlarge the balance thrust because of the difficulty in providing a sufficient high pressure to the high pressure balance chamber and a sufficient low pressure to the low pressure balance chamber. Those matters may readily be appreciated from the lines L.sub.46' and L.sub.48'. Those also means that the values of the specific peripheral speed K are set in the range of from 0.5 to 0.4 and at about 0 respectively. The causes of the above are that in the high pressure balance chamber 46 a relatively high speed rotation flow is generated thereby resulting in a rapid drop of the pressure of the treating liquid, while in the low pressure balance chamber 48 a relatively low speed rotation flow is generated thereby resulting in almost no variation of the pressure of the treating liquid.
The conventional thrust balance feature provides the high head and large capacity multistage pump with an insufficient balance thrust. To solve this problem, a large scale design modification is required of the pump. This may result in an increase of the manufacturing cost and in an increase of the friction loss.
As illustrated in FIG. 6, the multistage canned motor pump 10 comprises the multistage pump section 12 and the canned motor section 14 have the single rotor shaft 15 in common wherein the rotor shaft 15 is supported by a pair of bearings 17 and the circulation pipe 26 is provided for circulation of the treating liquid so that a part of the treating liquid is circulated within the motor section 14 for lubrication and cooling of the bearings 17 and the motor section 14.
As illustrated in FIG. 7, the adjacent pump chambers 13 constituting the pump section 12 are connected to each other through a fixed joint member 52 comprising an engaging hole between chamber walls 50 and through a slidable joint member 54 comprising a passage extending both along the rotor shaft 15 and between the chamber wall and a rotation boss 16a of the impeller 16. The fixed and slidable joint members 52 and 54 are provided with an O-ring 56 and an annular seal ring 58 respectively to prevent the leakage of the treating liquid from the high pressure pump stage to the low pressure pump stage.
The above described scaling structure has serious problems as described below. The conventional sealing structure comprises the fixed joint member 52 with the O-ring 56 and the slidable joint member 54 with the annular sealing member 58. The fixed joint member 52 is surely able to prevent any leakage by the O-ring 56, while the slidable joint member 54 makes it difficult to surely seal by the annular seal ring 58 because the annular seal ring 58 requires a large gap of sealing face as illustrated in FIG. 7, unlike the normal multistage pump.
In the multistage canned motor pump, the rotor shaft 15 is supported by a pair of the bearings 17 that are made of carbon or ceramic in place of alloys used in the normal multistage pump. The carbon or ceramic bearings is designed to have a large gap rather than that of the normal multistage pump due to a deterioration of the slidable property. The slidable joint member 54 constituting the scaling member between individual pump stages is designed to have a larger sealing area gap of the annular sealing 58 rather than the bearing area gap due to a large vibration of the rotor shaft 15. For that reason, the annular sealing member is obliged to have a poor sealing property.
The large sealing area gap may result in a poor sealing property of the annulare sealing member. A characteristic curve La' represented by a broken line in FIG. 23 shows that if a difference in pressure between individual stages of the pump section is raised, then an amount of the leakage q is also increased rapidly. A characteristic curve Lb' represented by a broken line in FIG. 24 shows that if a difference in pressure between individual stages of the pump section is raised, then the pump efficiency is lowered largely as compared to the normal multistage pump. The sealing area gap is enlarged by a frictional wear of the annulare sealing member 58 due to the eccentric vibration of the rotor shaft 15 caused by the frictional wear of the bearings 17. The increase of the gap may promote the lowering of the pump efficiency particularly due to the pump head and aged deterioration. The increase of the amount of the leakage q or the lowering of the pump efficiency may be somewhat prevented by enlarging the length in the axial direction of the annulare sealing member 58, but not may be prevented completely. In this case, an enlargement of the slidable point member 54 is required thereby the individual pump chamber 13 is also enlarged, resulting in an enlargement of the pump section 12.
The sealing structure between the individual pump stages in the canned motor pump is forced to be engaged with the problems with lowering of the pump efficiency due to a relatively large amount of the leakage and aged deterioration of the pump performances as well as the problem with an increase of the number of the pump stages.